The present invention relates to an elongate double-walled tube assembly, such as a concentric double-walled tube assembly, of thermoplastic resin particularly nylon, which tube is capable of passing a gas such as air or nitrogen or a fluid such as a liquid, and of being easily coupled to a connector or coupling for controlling operation of a fluid-pressure-operated device, and a method of continuously manufacturing such an elongate double-walled tube assembly with high dimensional accuracy and reproducibility through extrusion molding, using an extrusion molding machine having a cross-head die, and a sizing die device.
One conventional double-walled tube assembly of themoplastic resin and method of manufacturing such a double-walled tube assembly is disclosed in Japanese Patent Publication No. 61-029439, for example.
According to the disclosed manufacturing method, (a) two extrusion molding machines are used, (b) an inner tube and connecting ribs are separately formed by the first extrusion molding machine, (c) thereafter, the inner tube and the connecting ribs are welded and extruded together, (d) an outer tube is extruded by the second extrusion molding machine and is simultaneously joined to the unitary body of the inner tube and connecting ribs by a crosshead associated with the second extrusion molding machine, (e) so that a double-walled tube assembly of an integral structure is produced in which the outer tube and the connecting ribs are separable from each other and the connecting ribs and the inner tube are separable from each other. A double-walled tube assembly (concentric double-walled tube assembly) thus manufactured is illustrated in FIG. 1 of the accompanying drawings.
With the known manufacturing method, the inner tube 5 and the connecting ribs 7 are separately extruded as a unitary body by the first extrusion molding machine. Thereafter the outer tube 6 which is newly extruded by the second extrusion molding machine is melted and integrally joined to the inner tube 5 and the connecting ribs 7 by the crosshead for thereby producing a double-walled tube assembly 8 of thermoplastic resin. Therefore, when the ribs 7 are brought into contact with the melted outer tube 6, the ribs 7 are deformed because of the high temperature of the outer tube 6. Moreover, at the time the inner tube 5, the ribs 7, and the softened outer tube 6 are cooled and solidified after they have been extruded from the crossdie, the outer tube 6 shrinks radially inwardly toward the inner tube 5. As a consequence, double-walled tube assemblies with outer tubes of high dimensional accuracy cannot be manufactured by the conventional method.
In the known double-walled tube assembly 8, the inner tube 5 and the outer tube 6 are fused to each other by the ribs 7 at the opposite side edges of the ribs 7. Therefore, when a connector of a fluid-pressure-operated device is to be joined to the double-walled tube assembly by partly removing the outer tube 6 and the ribs 7 for only exposing a portion of the inner tube 5, it is necessary to separate the outer tube 6 from the ribs 7 at first, and then to separate the ribs 7 from the inner tube 5. Stated otherwise, in order to expose only a portion of the inner tube 5 when putting the double-walled tube assembly to use, an excessive separating process is required.
With the aforesaid conventional method, since the inner tube and the outer tube are independently extruded and then joined to each other, it would substantially be difficult to sufficiently reduce the width of the ribs and hence ribs of large width should be employed for preventing the ribs from being deformed at high temperature. Thus, the outer and inner tubes of the prior double-walled tube assembly are integrally joined to each other by the ribs of large width. The double-walled tube assembly is highly rigid, but not flexible enough to be freely curved or bent for use.